Gitlab and the 'jetbrains built in ssh library' do not work well together. The library wants to use SHA1 based key exchange algorithm where gitlab (default) only allows SHA2 based key exchange algorithm's.
The fact that it also might be the cause for your pull/push problem in IntelliJ, and also my push-problem in PhpStorm is based on the ASSUMPTION jetbrains use the same 'built in library' for all their software.
I had this issue with WebStorm recently (February/2018) and none of the (then) previous solutions worked for me. After spending some hours on troubleshooting and researching, I installed the 2018 EAP version and now it works! 2b1af7f3a8